Ionic conductors are formed of ceramic materials that are capable of conducting oxygen ions at elevated temperature and that have a low electronic conductivity. They are used to form electrolytes that are typically used within oxygen generators and solid oxide fuel cells. Such electrolytes are employed in a layered structure that has an anode and a cathode sandwiching the electrolyte. There are other uses for such materials known in the art such as steam electrolyzers and the like.
In case of an oxygen generator, when an electrical potential is applied across the anode and cathode, oxygen, in an oxygen containing feed, ionizes to produce oxygen ions which are transported through the electrolyte. The oxygen ions emerge from the electrolyte and recombine to form molecular oxygen. In a solid oxide fuel cell, the anode and cathode are connected to an electric load. A fuel is combusted using the permeated oxygen as an oxidizer. The electrons released as a result of the oxygen ions exiting the electrolyte at the anode travel to the electric load and then to the cathode to ionize the oxygen in the oxygen containing feed.
Oxygen generators, solid oxide fuel cells and like devices use elements having layered anode-electrolyte-cathode structures in the form of flat plates or tubes that are fabricated by known techniques such as isostatic pressing and tape casting. In such methods oxygen ion conducting materials such as doped zirconia or gadolinium doped ceria in the form of a powder are mixed with an organic binder and then molded into the desired shape or onto the anode layer. The anode layer can be a conductive metal such as silver supported by an inert structure or a mixed conductor capable of conducting both oxygen ions and electrons. The resultant green form is fired to burn out the binder and to sinter the materials into a coherent mass. Thereafter, the cathode layer is applied.
As may be appreciated, to be useful in a practical device, such as an oxygen generator or a solid oxide fuel cell, it is important that the oxygen ion conductivity be the maximum obtainable for a particular material as well as its sintered density and therefore the strength thereof. As mentioned in 106 Solid State Ionics, “Properties of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) Double Layer Cathodes on Gadolinium-Doped Cerium Oxide (CGO) Electrolytes I. Role of SiO2”, by Bae et al., pp. 247-253 (1998), silicon in the form of silicon oxides has a ubiquitous presence in all oxides. Such silicon can negatively influence the conductivity of an electrolyte formed of gadolinium-doped cerium oxide (hereinafter “CGO”).
The need to increase the conductivity of CGO electrolytes, particularly at low temperatures, has been identified in the prior art with respect to solid oxide fuel cells. CGO, while having a high conductivity, is not robust in high temperature reducing atmospheres present in solid oxide fuel cells. Hence, it is necessary to use the CGO for such applications at relatively low temperatures of operation in the neighborhood of 500° C. to 700° C. Moreover, the cost of the solid oxide fuel cell is also reduced by operating it at a lower temperature because less temperature critical components are required. However, at such temperatures, the oxygen ion conductivity becomes particularly critical for CGO. Hence, there exists the need in such applications and operations to maximize the conductivity of CGO.
In 575 Material Research Society Symposium Proceedings, “Improving Gd-Doped Ceria Electrolytes for Low Temperture Solid Oxide Fuel Cells”, by Ralph et al., pp. 309-314 (2000), the conductivity of CGO having impurities such as silicon dioxide is improved by doping the CGO with calcium. It is suggested in this reference that praseodymium and iron dopants would have the same effect. The calcium-doped CGO having impurities is made by using an amorphous citrate route of preparation. Such preparation is an atomic mixing technique that involves mixing cation salts in proper stoichiometric ratios with citric acid and then dissolving the resultant mixture in water to produce an aqueous solution. The solution is then heated and calcined to form the oxide.
In Ralph et al., it is mentioned that grain boundary conductivities showed an improvement over the standard CGO samples due to formation of a second phase of reasonably good conductivity as compared with the poor conductivity of the impurity oxides such as SiO2. In Ralph et al. the SiO2 concentration is stated to be less than 20 parts per million.
In 129 Solid State Ionics, “Appraisal of Ce1-yGdyO2-y/2 Electrolytes for IT-SOFC Operation at 500° C.”, by Steele, pp. 95-110 (2000), it is noted that the use of highly purified powders for CGO and doped zirconia electrolytes, that is an SiO2 content of less than 50 parts per million in order to obtain sufficient conductivity of the electrolyte material at low temperatures of operation.
It therefore can be understood from the foregoing references that contaminants such as silicon in the form of silicon oxides act to lower ionic conductivity in CGO and doped zirconia electrolyte materials. In order to operate an SOFC employing an electrolyte formed of CGO and other materials at low temperature, it is necessary that the ionic conducting material making up the electrolyte should be as pure as possible, that is contain a minimum amount of silicon. Furthermore, such pure forms of CGO can be doped to provide a further increase in low temperature conductivity with the use of calcium dopants. As may be appreciated, the same criteria for the use of CGO and YSZ in solid oxide fuel cells applies equally to other similar devices such as oxygen generators.
In US 2001/0007381 A1, a salt solution containing a transition metal dopant, for instance, iron dissolved in a solution, is applied to purified CGO powder in an amount of about 2 mol %. This treatment reduces the sintering temperature so that a sintered ceramic element with small grain size can be produced having superior strength to untreated CGO.
It is to be also noted that the purer the electrolyte powder, the higher the costs involved in obtaining the electrolyte. For instance, a powder 99 percent pure costs about 75 percent as much as a powder 99.9 percent pure which in turn costs about 60 percent as much as a powder 99.99 percent pure. Hence, while there exists the general need to raise the oxygen ion conductivity of ion conductors having grain boundary impurities such as silicon or silicon containing compounds, such need is particular acute with particularly low purity oxygen ion conducting materials. If such materials can be made useful by enhancing their ion conductivity, they are particularly advantageous due to their low cost.
As will be discussed, the present invention provides a method of manufacturing ionic conductor materials that are doped with alkaline-earth metals that enhances the oxygen ion conductivity over that obtainable by prior art manufacturing techniques. In this regard, as will be discussed, such prior art techniques, such as disclosed in the Ralph et al. article are not effective in enhancing the conductivity of low purity ionic conductors such as CGO. Furthermore, an added benefit of the present invention is that the strength of the ionic conducting material is also enhanced.